IP Multicast over Token-Ring Local Area Networks (RFC1469)
Original Publication Date: 1993-Jun-01
Included in the Prior Art Database: 2019-Feb-10
Internet Society Requests For Comment (RFCs)
This document specifies a method for the transmission of IP multicast datagrams over Token-Ring Local Area Networks. [STANDARDS-TRACK]
Network Working Group T. Pusateri Request for Comments: 1469 Consultant June 1993
IP Multicast over Token-Ring Local Area Networks
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This document specifies a method for the transmission of IP multicast datagrams over Token-Ring Local Area Networks. Although an interim solution has emerged and is currently being used, it is the intention of this document to specify a more efficient means of transmission using an assigned Token-Ring functional address.
IP multicasting provides a means of transmitting IP datagrams to a group of hosts. A group IP address is used as the destination address in the IP datagram as documented in STD 5, RFC 1112 . These group addresses, also referred to as Class D addresses, fall in the range from 184.108.40.206 to 220.127.116.11. A standard method of mapping IP multicast addresses to media types such as ethernet and fddi exist in  and RFC 1188 . This document attempts to define the mapping for an IP multicast address to the corresponding Token- Ring MAC address.
The Token-Ring Network Architecture Reference  provides several types of addressing mechanisms. These include both individual (unicast) and group addresses (multicast). A special subtype of group addresses are called functional addresses and are indicated by a bit in the destination MAC address. They were designed for widely used functions such as ring monitoring, NETBIOS, Bridge, and Lan Manager frames. There are a limited number of functional addresses, 31 in all, and therefore several unrelated functions must share the same functional address.
Pusateri [Page 1]
RFC 1469 IP Multicast over Token-Ring LANs June 1993
It would be most desirable if Token-Ring could use the same mapping as ethernet and fddi for IP multicast to hardware multicast addressing. However, current implementations of Token-Ring controller chips cannot support this. To see why, we must first examine the Destination MAC address format.
Destination Address Format
The destination MAC address consists of six octets. In the following diagram of a MAC address, the order of transmission of the octets is from top to bottom (octet 0 to octet 5), and the order of transmission of the bits within each octet is from right to left (bit 0 to bit 7). This is the so-called "canonical" bit order for IEEE 802.2 addresses. Addresses supplied to or received from token ring interfaces are usually laid out in memory with the bits of each octet in the opposite order from that illustrated, i.e., with bit 0 in the high-order (leftmost) position within the octet.
7 6 5 4 3 2 1 0
--------------------------------- | | | | | | |U/L|I/G| octet 0 --------...